1. Technical Field
The present disclosure is directed to electrosurgical systems, and, in particular, to an electrosurgical system for vessel sealing using capacitive radio frequency (RF) dielectric heating.
2. Description of the Related Art
Electrosurgical generators are employed by surgeons in conjunction with an electrosurgical tool to cut, coagulate, desiccate and/or seal patient tissue. High frequency electrical energy, e.g., radio frequency (RF) energy, is produced by the electrosurgical generator and applied to the tissue by the electrosurgical tool. Both monopolar and bipolar configurations are commonly used during electrosurgical procedures.
Electrosurgical generators typically include power supply circuits, front panel interface circuits, and RF output stage circuits. Many electrical designs for electrosurgical generators are known in the field. In certain electrosurgical generator designs, the RF output stage can be adjusted to control the RMS (root mean square) output power. The methods of controlling the RF output stage may include changing the duty cycle, or changing the amplitude of the driving signal to the RF output stage. The method of controlling the RF output stage is described herein as changing an input to the RF output stage.
Electrosurgical techniques have been used to seal or fuse small diameter blood vessels, vascular bundles and tissue. In this application, two layers of tissue are grasped and clamped together while electrosurgical power is applied. By applying a unique combination of pressure, gap distance between opposing seal surfaces and controlling the electrosurgical energy, the two tissue layers are welded or fused together into a single mass with limited demarcation between tissue layers. Tissue fusion is similar to vessel sealing, except that a vessel or duct is not necessarily sealed in this process. For example, tissue fusion may be used instead of staples for surgical anastomosis.
One of the issues associated with electrosurgical sealing or fusion of tissue is undesirable collateral damage to tissue due to the various thermal effects associated with electrosurgically energizing tissue. The tissue at the operative site is heated by electrosurgical current typically applied by the electrosurgical instrument. Healthy tissue adjacent to the operative site may become thermally damaged if too much heat is allowed to build up at the operative site or adjacent the sealing surfaces. For example, during sealing, the heat may conduct or spread to the adjacent tissue and cause a significant region of tissue necrosis. This is known as thermal spread. Thermal spread becomes important when electrosurgical instruments are used in close proximity to delicate anatomical structures. Therefore, an electrosurgical generator that reduces the possibility of thermal spread would offer a better opportunity for a successful surgical outcome.
Another issue associated with electrosurgical tissue sealing or tissue fusion is the buildup of eschar on the surgical instrument. Eschar is a deposit which is created from tissue that is charred by heat. Surgical tools often lose effectiveness when coated with eschar.
In order to effect a proper seal with larger vessels, two predominant mechanical parameters must be accurately controlled—the pressure applied to the vessel and the gap between the electrodes both of which affect thickness of the sealed vessel. More particularly, accurate application of the pressure is important to oppose the walls of the vessel, to reduce the tissue impedance to a low enough value that allows enough electrosurgical energy through the tissue, to overcome the forces of expansion during tissue heating and to contribute to the end tissue thickness which is an indication of a good seal. It has been determined that a fused vessel wall is optimum between about 0.001 and about 0.006 inches. Below this range, the seal may shred or tear and above this range the lumens may not be properly or effectively sealed.
With respect to smaller vessels, the pressure applied to the tissue tends to become less relevant whereas the gap distance between the electrically conductive surfaces becomes more significant for effective sealing. In other words, the chances of the two electrically conductive surfaces touching during activation increases as the vessels become smaller.
As mentioned above, in order to properly and effectively seal larger vessels, a greater closure force between opposing jaw members is required. It is known that a large closure force between the jaws typically requires a large moment about the pivot for each jaw. This presents a challenge because the jaw members are typically affixed with pins which are positioned to have a small moment arms with respect to the pivot of each jaw member. A large force, coupled with a small moment arm, is undesirable because the large forces may shear the pins. As a result, designers must compensate for these large closure forces by either designing instruments with metal pins and/or by designing instruments which at least partially offload these closure forces to reduce the chances of mechanical failure. As can be appreciated, if metal pivot pins are employed, the metal pins must be insulated to avoid the pin acting as an alternate current path between the jaw members which may prove detrimental to effective sealing.
Increasing the closure forces between electrodes may have other undesirable effects, e.g., it may cause the opposing electrodes to come into close contact with one another which may result in a short circuit or arcing between the electrodes, and a small closure force may cause pre-mature movement of the tissue during compression and prior to activation.
Thus, a need exists to develop an electrosurgical system which effectively seals vascular tissue and solves the aforementioned problems by providing an instrument which enables a large closure force between the opposing jaws members, reduces the chances of short circuiting the opposing jaws during activation and reduces the possibility of thermal spread.